1. Field of the Invention
The present invention relates to an electric control apparatus for a transfer device in part-time four-wheel drive vehicles, the transfer device being arranged to be switched over by operation of a manual operation switch to selectively provide a two-wheel drive mode or a four-wheel drive mode of operation for the vehicle.
2. Discussion of the Prior Art
In Japanese Utility Model Early Publication No. 63-69034, there has been proposed a part-time four-wheel drive system which includes a transfer device drivingly connected to a prime mover of the vehicle to continuously transfer the power from the prime mover to a set of rear road wheels and to selectively transfer the power to a set of front road wheels under control of a clutch mechanism provided therein, and a front differential in drive connection to split axle parts of the front road wheels at its output side and being drivingly connected to the transfer device at its input side to selectively transmit the power from the transfer device to the front road wheels under control of a clutch mechanism provided therein. In the part-time four-wheel drive system, each actuator of the clutch mechanisms in the transfer device and differential is activated under control of a manual operation switch. When the operation switch is manipulated by the driver to select a four-wheel drive mode, both the clutch mechanisms are engaged by activation of their associated actuators to transmit the power to the front road wheels through the transfer device and differential. When the operation switch is manipulated by the driver to select a two-wheel drive mode, both the clutch mechanisms are disengaged by activation of their associated actuators to disconnect the differential from the transfer device and from the split axle parts of the front road wheels. This is effective to eliminate back drive of the differential and propeller shaft for the non-driven front road wheels.
In the transfer device, a synchronizer is assembled with the clutch mechanism to establish synchronization between drive and driven members in the transfer device when the clutch mechanism is brought into engagement to select the four-wheel drive mode. The component parts of the synchronizer are applied with a heavy load from the driven member if the clutch mechanism is engaged in a condition where the viscous resistance of lubricating oil in the differential is increased due to drop of the ambient temperature to cause an increase of drag resistance of the driven member or in a condition where a difference in rotational speed between the drive and driven members is large during high speed travel of the vehicle. This results in defacement of the component parts of the synchronizer in a short period of time.